Recent requirements have led to downsizing a torque converter, for example by shortening a diameter of a torque converter, or by shortening an axial thickness thereof. As a result, a narrow space, at which a torque converter is assembled, a narrow space, at which a torque converter is mounted on an automobile, a motorbike, and so on, a narrow space for housing a torque converter, and so on can be effectively assured. Further, as a result, a manufacturing cost of a torque converter can be effectively reduced.
As one of necessary requirements for attaining a downsizing of a torque converter, a torque capacity coefficient C, which is one of characteristics of a torque converter, has to be improved over an entire range of a speed ratio e. The torque capacity coefficient C is determined by the following equation (1):C=Ti/Ni2  (1), andthe speed ratio e is determined by the following equation (2):e=No/Ni  (2).
Herein “Ti” represents an input torque, “Ni” represents an input shaft rotational number, and “No” represents an output shaft rotational number. An improvement in a torque capacity coefficient C can be effectively obtained in favor of elaborate designs of a torque converter, i.e., by achieving an optimal design of a torque converter, in terms of a shape of each blade (pump blade, turbine blade, and stator blade), a design of a torus outer shell, and so on.
Recently, complicated blade shape of a torque converter has been optimized. For example according to JP 1980 (55)-027598 (corresponding to U.S. Pat. No. 4,260,330), JP1990 (02)-278052A, a designing of a blade shape for the purpose of enhancing a torque capacity coefficient C has attained some positive results.
At variance with improvements in performances of a personal computer, however, according to present design technologies, it is still not possible to put forward required characteristics of a torque converter, and further to satisfy design requirements for producing a complicated torque converter blade shape. In the present studies, materials disclosed remain confined to only the two-dimensional analysis of a stator blade.
Therefore, there may on occasions be designing procedures of designers for designing blade shapes, designing procedures which adopt a one-dimensional performance analysis disclosed in “Design of Single-stage, Three-element Torque Converter” (V. J. Jandasek, SAE Technical Papers Document, No. 610576, pp. 208-209), and in “Torque converter” (Tomoo Ishihara, Report of Institute of Industrial Science, University of Tokyo, 30 Mar. 1955, Volume 5, Edition 7: 85-86). According to this one-dimensional performance analysis, an exit angle of a pump blade is designed at, or greater than, −50 degrees as an optimal angle. If the pump blade is designed to be less than this optimal angle, −50 degrees, an assumption on the basis of this one-dimensional performance analysis may not be able to be established. According to this one-dimensional performance analysis, an angle limit of an optimal angle of the pump blade is −68 degrees. Therefore, it has been unrealistic to design an exit angle of the pump blade at, or less than, −68 degrees. This optimal angle of the pump blade is expressed with a minus sign (−) which represents the pump blade being inclined in the same direction as a crankshaft rotational direction.
Moreover, in terms of an object of downsizing a torque converter, while a procedure to improve a torque capacity coefficient is being pursued, there has been a danger that a torque capacity coefficient within a range of a middle speed ratio (speed ratio e=0.3 to 0.7) becomes greater than a torque capacity coefficient within a range of a low speed ratio (speed ratio e=0 to 0.3). When a torque converter with a superior torque capacity coefficient is mounted on a vehicle such as an automobile, this sort of danger on occasions causes reduction in an engine rotational speed at the low speed ratio range, for example when a vehicle is started or accelerated, thereby damaging an accelerating feeling. This unfavorable circumstance occurs, because, if a torque capacity coefficient is designed at a relatively large value within the range of the middle speed ratio, a flow velocity of a working fluid within the middle speed ratio range becomes greater than a fluid velocity in a circulating direction of a working fluid within the low speed ration range.
Meanwhile, recent developments have led to various experiments which relate to developments in a three-dimensional shape measuring apparatus, and improvements in a press molding precision. In details, recently, it is possible to assess a complicated blade structure by way of a three-dimensional analysis, and is possible to optimize a compressing ratio of a torus-shaped outer shell, by which a friction loss, and an impact loss, inside a torque converter can be effectively reduced. Attention of inventors of the present invention is focused on changing parameters of a blade shape, a change which has conventionally been considered to be unrealistic, thereby enabling to improve a torque transmitting efficiency of a torque converter.
The present invention has been made in view of the above circumstances, and provides a torque converter which can be downsized, and which is capable of improve an accelerating feeling within a range of a low speed ratio, i.e., at a time that a vehicle is started.